Objetivos pedagógicos

To present the different parameters used to judge the quality of an image

Describe the factors influencing the signal-to-noise ratio and their interdependence

List the different MRI artifacts, their origin, effects on the image and ways of reducing them:

Movements, phantom images, flow

Magnetic susceptibility and metal artifacts

Truncation / Gibb’s

Folding / aliasing

Chemical shift

Cross-excitation

Magic angle

Present the basic criteria in MRI quality control

Pontos principais

Signal to noise ratio

Principles

Compromise between:

Spatial resolution: limited by the voxel size which is determined by the matrix size, the field of view and slice thickness

Signal to noise ratio: depending on the voxel size, the number of averagings and the receiver bandwidth

Total scan time

Which also modify the available sequence parameters (TE) and the artifacts.

Remedies (and penalties)

The signal to noise ratio goes up

Penalty

With the voxel size

Limits the spatial resolution

When a local or surface coil is used instead of a large or body coil

Reduced field of exploration

With the number of averagings (square root factor)

Longer scan time

When the receiver bandwidth decreases (square root factor)

Higher chemical shift artifacts

Motion and ghosting

Principles

Motion is responsible for a corruption in spatial localization in the phase-encoding direction, resulting in a blur and ghost images propagated in the phase-encode direction.

Remedies (and penalties)

Remedies

Penalty

Reduce body movements (physical restraint, sedation)

Patient's comfort

Breath-hold sequences

Requires apnea and fast sequences

Gating and movement compensation

Increase the scan time, restrict the available TRs, a blur persists with some methods

Signal suppression of moving tissues (fat of abdominal wall...)

Increased TR due to magnetization preparation or saturation bands

Swapping phase-encode and frequency-encode directions

Only shifts the artifacts, consequences on sequence parameters and other artifacts

Magnetic susceptibility

Principles

At the interface between two tissues with different magnetic susceptibilities, there are local distortions in the magnetic field responsible for a signal loss (and sometimes an image distortion). These artifacts are much stronger in presence of metal.

Remedies (and penalties)

Remedies

Penalty

SE rather than GE sequences

Short TE

Restricts available contrasts

Increased receiver bandwidth to reduce the minimal TE

Decreased SNR

Usage

detection of hematomas (blood breakdown products)

quantification of liver iron content

detection of liver metastases

perfusion imaging

Truncation

Principles

The image is reconstructed from k-space by a 2D inverse Fourier transform. As k-space data are finite and defined by the matrix size, the reconstruction of high-contrast interfaces is imperfect and causes visible artifacts. These artifacts appear as multiple parallel lines adjacent to the interface or as false widening of the edges at this interface. Truncation artifacts can occur in both the frequency and phase-encode directions.

Remedies (and penalties)

Remedies

Penalty

Incread matrix size

Increased scan time Decreased SNR

Aliasing

Principles

Aliasing or wrap-around results from a spatial mismapping caused by an undersampling in the phase-encode direction. Consequently, objects outside of the FOV overlap on the opposite side of the image.

Remedies (and penalties)

Remedies

Penalty

Swapping the frequency-encode and phase-encode directions

Increasing the FOV

Decreased spatial resolution

Phase oversampling

Increased scan time

No-phase wrap or Anti-aliasing

Decreased SNR

Chemical shift

Principles

The chemical environment of protons can cause a shift in their precessional frequency. The shift in resonance frequency between protons in fat and water is roughly equal to 3.5 ppm, corresponding to a difference of about 225 Hz at 1.5 T.

This frequency shift is responsible for a spatial misregistration in the frequency-encode direction, resulting in a contour artifact with dark and white bands at the interfaces between fat and tissues in the frequency-encode direction : the chemical shift artifact of the first kind. As the resonance frequency shift causes a phase shift which is not canceled in gradient echo sequences, there is a black line at all fat/tissue borders when fat and water are out of phase (TE = 2.2 ms at 1.5 T) : the chemical shift artifact of the second kind. This artifact occurs in both the frequency-encode and phase-encode directions.